This invention relates to a resin composition, and a fuel cell separator and sealing material each comprising the resin composition.
In recent years, there is an increasing demand for a fuel cell capable of directly changing the chemical energy of a fuel to electric energy.
A fuel cell is generally formed of a plurality of unit cells stacked one after another and each unit cell has electrode plates having an electrolyte-containing matrix sandwiched therebetween and a separator disposed outside the electrodes. Since a fuel is usually supplied to one side of the separator, while an oxidizing gas or the like is supplied to the other side, the separator is required to have excellent impermeability to gases so as to prevent mixing of these two substances. In addition, the separator is required to have high electrical conductivity and at the same time has excellent strength, because the fuel cell is composed of stacked unit cells.
As a fuel cell separator, conventionally employed is a molded product obtained by press molding a graphite sheet, a resin-impregnated material comprising a carbon sintered body impregnated with a resin, a glassy carbon available by sintering a thermosetting resin in an inert atmosphere, or a resin molded product obtained by molding a mixture of carbon powders and a resin.
For example, disclosed in JP-A-58-53167, JP-A-60-37670, JP-A-60-246568, JP-B-64-340, JP-B-6-22136 or WO97/02612 (the terms xe2x80x9cJP-Axe2x80x9d and JP-Bxe2x80x9d as used herein mean an unexamined Japanese patent application and an examined published Japanese patent publication, respectively) is a separator composed of a thermosetting resin such as phenol resin, graphite and carbon; in JP-B-57-42157 a bipolar separator composed of a thermosetting resin such as epoxy resin and an electrically conductive substance such as graphite; in JP-A-1-311570 a separator obtained by incorporating expanded graphite and carbon black in a thermosetting resin such as phenol resin or furan resin; and in JP-A-8-259767 an electrically conductive plastic plate obtained by incorporating carbon black in an ethylene-ethyl acetate copolymer or the like.
In JP-A-8-31231, disclosed is a molded product obtained by incorporating ketjen black and spherical graphite in a thermoplastic or thermosetting resin and as the resin, an aromatic polyimide or the like is described.
Fillers such as graphite and carbon black can also be used as raw materials for a sealing material. For example, the filling of a polymer such as rubber with carbon black improves the swelling degree of the polymer in a solvent and at the same time, imparts it with properties such as strength and abrasion resistance. Graphite, alone or as a composite with a polymer, is used as a sealing material such as packing or gasket because of its excellent sliding properties and fitness to the surface of the flange.
When a resin-impregnated material is used as a fuel cell separator, cutting is required in order to form a channel (groove) for the circulation of a fuel gas or the like, which inevitably increases the labor and cost upon manufacture. Although the use of glassy carbon permits molding into a product shape before sintering, a problem in dimensional stability such as dimensional shrinkage occurs upon sintering. A resin molded product is, on the other hand, accompanied with such a merit as easy molding, but is inferior in electrical conductivity because of electrical insulation properties of the resin. Filling of a large amount of a filler such as carbon powder in the resin in order to improve electrical conductivity however makes it difficult or impossible to mold or form the resulting resin.
Filling of a large amount of a filler brings about various advantages to a sealing material. For example, gas permeation resistance can be improved by filling a large amount of carbon black in a rubber. Particularly, in a graphite type sealing material, it is desired to heighten the mixing ratio of graphite as much as possible to attain high sliding properties and surface fitness.
Filling of a large amount of a filler, however, causes problems such as deterioration in moldability.
An object of the invention is to provide a resin composition which can contain a large amount of a filler, has excellent moldability and can provide a molded product having both high electrical conductivity and mechanical strength; and a fuel cell separator and a sealing material prepared by molding the above-described resin composition.
The present invention provides a resin composition comprising A) 100 parts by weight of a resin composed of 0 to 99 parts by weight of an epoxy resin and 1 to 100 parts by weight of a polyimide resin, with the proviso that the total amount of the epoxy resin and the polyimide resin is 100 parts by weight, and B) 40 to 900 parts by weight of at least one filler selected from the group consisting of graphite, ketjen black, acetylene black, furnace carbon black and thermal carbon black.